Recombinant DNA technology is used for production of heterologous proteins in various host microorganisms and animals including Escherichia coli (hereinafter referred to as E. coli). The target products are various biogenous proteins (herein, inclusive of polypeptides), and many of them have already been produced industrially for medical and other uses so far.
Among various hosts developed for production of heterologous proteins, yeasts seem favorable for expression of animal and plant proteins because of their eukaryotic similarity in the transcription and translation systems to animals and plants, and the baker's yeast (Saccharomyces cerevisiae) is a widely used host. Among yeasts, S. pombe is known to be close to animal cells in nature as is evident from the fact that it grows by fission not by budding as a result of the different evolution process it has followed since it diverged from other yeasts at early stages. Therefore, the use of S. pombe as the host for expression of heterologous proteins is expected to provide a gene product closer to its natural form in animal cells.
Though studies of gene expression in S. pombe is delayed, the recent discovery of potent promoters functional in S. pombe has accelerated the development of expression systems using S. pombe as the host, and various improvements have been added to expression vectors to develop more stable and efficient expression systems (Japanese Patent No. 2776085, JP-A-07-163373, JP-A-10-215867, JP-A-10-234375, JP-A-11-192094, JP-A-2000-136199, JP-A-2000-262284). As a result, expression systems using S. pombe as the host show high production efficiency now.
Production systems for heterologous proteins using eukaryotic microorganisms such as yeasts can be realized easily by conventional microbiological techniques and recombinant DNA technology with high productivity. Large cultures are already available and are acceleratingly used for actual production. Even after the scale is enlarged for actual production, cells retain the high production efficiency per cell obtained in the laboratory.
However, considering that cost reduction is often demanded in actual production, it is necessary to improve the production efficiency of heterologous proteins through improvement in cell growth efficiency, suppression of degradation of the heterologous protein of interest, more efficient eukaryotic modifications in the microorganisms or more efficient utilization of the nutrition sources. For example, increase in the conversion of the carbon sources added to the medium for culture growth into the heterologous protein of interest is expected to drastically improve cell growth efficiency and therefore production efficiency of the heterologous protein, because efficient utilization of the carbon sources in the medium for production of the heterologous protein of interest seems to be sacrificed for their consumption by metabolic systems unnecessary for cell growth or production of the heterologous protein of interest (such as the ethanol fermentation system for production of ethanol).